Ultrasound-Doppler assessment of diastolic function is subject to velocity errors caused by angle sensitivity and a fixed location of the sample volume. We used 3-dimensional phase contrast magnetic resonance imaging (MRI) to evaluate these errors in 10 patients with hypertension and in 10 healthy volunteers. The single (Doppler) and triple (MRI) component velocity was measured at early (E) and late (A) inflow along Doppler-like sample lines or 3-dimensional particle traces generated from the MRI data. Doppler measurements underestimated MRI velocities by 9.4% +/- 8.6%; the effect on the E/A ratio was larger and more variable. Measuring early and late diastolic inflows from a single line demonstrated the error caused by their 3-dimensional spatial offset. Both errors were minimized by calculating the E/A ratio from maximal E and A values without constraint to a single line. Alignment and spatial offset are important sources of error in Doppler diastolic parameters. Improved accuracy may be achieved with the use of maximal E and A velocities from wherever they occur in the left ventricle.